The present invention relates to a television signal encoder and more particularly, to a clamp level setting method for TV signals appropriate for a TV signal encoder having a plurality of quantizers.
In a TV signal encoder, an analog TV signal is converted into a digitized TV signal by means of an A/D converter. The digitized TV signal then undergoes highly efficient encoding, such as a differential pulse code modulation (DPCM). Since an A/D converter generally has a predetermined dynamic range, the level of the TV signal is limited or clamped to a predetermined level prior to the A/D conversion. The clamping is performed by adding a DC offset to the analog TV signal, so that the fixed level of the TV signal, such as a sync tip level (the level at the start of a synchronization signal) or pedestal level (black level), assumes a predetermined level (clamp level). Conventionally, the clamp level is chosen to place the entire TV signal within the dynamic range of an A/D converter. On the other hand, several kinds of TV signal encoders have been developed, and one of them is a TV signal encoder which comprises a plurality of quantizers each having a different quantizing characteristic. By selectively switching the quantizers, coded information to be generated by the encoder is controlled. As more precisely described later, if the conventional clamp level setting method is applied to that TV signal encoder, the sync tip level, for example, may be quantized to another quantizing level which is different from the original level to which the sync tip level is quantized. The error thus caused by the quantizer appears on a display, such as television monitor, in the form of uneven brightness. This is due to the fact that the display utilizes the sync tip level as a reference for the display's brightness.